gerrit: Support gerrit-trigger >= 2.6.0.

Gerrit-trigger offers different triggers, stored in config file using
different formats.
During development of version 2.6.0, the following markup was used[1]:

  <triggerOnPatchsetUploadedEvent>true</triggerOnPatchsetUploadedEvent>
  <triggerOnChangeMergedEvent>true</triggerOnChangeMergedEvent>
  <triggerOnCommentAddedEvent>false</triggerOnCommentAddedEvent>
  <triggerOnRefUpdatedEvent>false</triggerOnRefUpdatedEvent>

That markup changed before the release of 2.6.0 [2] to:

  <triggerOnEvents>
    <com.sonyericsson.hudson.plugins.gerrit.trigger.hudsontrigger.events.PluginPatchsetCreatedEvent/>
    <com.sonyericsson.hudson.plugins.gerrit.trigger.hudsontrigger.events.PluginChangeRestoredEvent/>
  </triggerOnEvents>

Adjust the generated XML tree to match what gerrit-trigger expects.

This means that gerrit-trigger versions < 2.6.0 are no longer supported.

Trigger on draft published, added in 2.6.0, is now supported.

[1] Commit 8bee93a87b59d413ccec0aa84d3da10adb108eb9
[2] Commit 9ba008972a84e3ca8b2a90bd6d21295e7d7f4f94

Change-Id: I7160de1284c53d20f435e5af0d5a64b2e17c4196
Reviewed-on: https://review.openstack.org/24263
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Anita Kuno <akuno@lavabit.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins